ASYMMETRIC-KEY(7) Asymmetric Kernel Key Type ASYMMETRIC-KEY(7)
asymmetric - Kernel key type for holding asymmetric keys
A kernel key of asymmetric type acts as a handle to an asymmetric key
as used for public-key cryptography. The key material itself may be
held inside the kernel or it may be held in hardware with operations
being offloaded. This prevents direct user access to the cryptographic
Keys may be any asymmetric type (RSA, ECDSA, ...) and may have both
private and public components present or just the public component.
Asymmetric keys can be made use of by both the kernel and userspace.
The kernel can make use of them for module signature verification and
kexec image verification for example. Userspace is provided with a set
of keyctl(KEYCTL_PKEY_*) calls for querying and using the key. These
are wrapped by libkeyutils as functions named keyctl_pkey_*().
An asymmetric-type key can be loaded by the keyctl utility using a com-
mand line like:
openssl x509 -in key.x509 -outform DER |
keyctl padd asymmetric foo @s
The asymmetric-type key can be viewed as a container that comprises of
a number of components:
The asymmetric key parsers attempt to identify the content of
the payload blob and extract useful data from it with which to
instantiate the key. The parser is only used when adding, in-
stantiating or updating a key and isn't thereafter associated
with the key.
Available parsers include ones that can deal with DER-encoded
X.509, DER-encoded PKCS#8 and DER-encoded TPM-wrapped blobs.
Public and private keys
These are the cryptographic components of the key pair. The
public half should always be available, but the private half
might not be. What operations are available can be queried, as
can the size of the key. The key material may or may not actu-
ally reside in the kernel.
In addition to the normal key description (which can be gener-
ated by the parser), a number of supplementary identifiers may
be available that can be searched for. These may be obtained,
for example, by hashing the public key material or from the sub-
jectKeyIdentifier in an X.509 certificate.
Identifier-based searches are selected by passing as the de-
scription to keyctl_search() a string constructed of hex charac-
ters prefixed with either "id:" or "ex:". The "id:" prefix in-
dicates that a partial tail match is permissible whereas "ex:"
requires an exact match on the full string. The hex characters
indicate the data to match.
This is the driver inside the kernel that accesses the key mate-
rial and performs operations on it. It might be entirely soft-
ware-based or it may offload the operations to a hardware key
store, such as a TPM.
Note that expiry times from the payload are ignored as these patches
may be used during boot before the system clock is set.
The asymmetric key parsers can handle keys in a number of forms:
X.509 DER-encoded X.509 certificates can be accepted. Two identifiers
are constructed: one from from the certificate issuer and serial
number and the other from the subjectKeyIdentifier, if present.
If left blank, the key description will be filled in from the
subject field plus either the subjectKeyIdentifier or the seri-
alNumber. Only the public key is filled in and only the encrypt
and verify operations are supported.
The signature on the X.509 certificate may be checked by the
keyring it is being added to and it may also be rejected if the
key is blacklisted.
PKCS#8 Unencrypted DER-encoded PKCS#8 key data containers can be ac-
cepted. Currently no identifiers are constructed. The private
key and the public key are loaded from the PKCS#8 blobs. En-
crypted PKCS#8 is not currently supported.
DER-encoded TPM-wrapped TSS key blobs can be accepted. Cur-
rently no identifiers are constructed. The public key is ex-
tracted from the blob but the private key is expected to be res-
ident in the TPM. Encryption and signature verification is done
in software, but decryption and signing are offloaded to the TPM
so as not to expose the private key.
This parser only supports TPM-1.2 wrappings and enc=pkcs1 encod-
ing type. It also uses a hard-coded null SRK password; pass-
word-protected SRKs are not yet supported.
In addition to the standard keyutils library functions, such as
keyctl_update(), there are five calls specific to the asymmetric key
type (though they are open to being used by other key types also):
The query function can be used to retrieve information about an asym-
metric key, such as the key size, the amount of space required by buf-
fers for the other operations and which operations are actually sup-
The other operations form two pairs: encrypt/decrypt and create/verify
signature. Not all of these operations will necessarily be available;
typically, encrypt and verify only require the public key to be avail-
able whereas decrypt and sign require the private key as well.
All of these operations take an information string parameter that sup-
plies additional information such as encoding type/form and the pass-
word(s) needed to unlock/unwrap the key. This takes the form of a
comma-separated list of "key[=value]" pairs, the exact set of which de-
pends on the subtype driver used by a particular key.
Available parameters include:
The encoding type for use in an encrypted blob or a signature.
An example might be "enc=pkcs1".
The name of the hash algorithm that was used to digest the data
to be signed. Note that this is only used to construct any en-
coding that is used in a signature. The data to be signed or
verified must have been parsed by the caller and the hash passed
to keyctl_pkey_sign() or keyctl_pkey_verify() beforehand. An
example might be "hash=sha256".
Note that not all parameters are used by all subtypes.
An additional keyutils function, keyctl_restrict_keyring(), can be used
to gate a keyring so that a new key can only be added to the affected
keyring if (a) it's an asymmetric key, (b) it's validly signed by a key
in some appropriate keyring and (c) it's not blacklisted.
Where <signing-key> is the ID of a key or a ring of keys that act as
the authority to permit a new key to be added to the keyring. The
chain flag indicates that keys that have been added to the keyring may
also be used to verify new keys. Authorising keys must themselves be
asymmetric-type keys that can be used to do a signature verification on
the key being added.
Note that there are various system keyrings visible to the root user
that may permit additional keys to be added. These are typically gated
by keys that already exist, preventing unauthorised keys from being
used for such things as module verification.
When the attempt is made to add a key to the kernel, a hash of the pub-
lic key is checked against the blacklist. This is a system keyring
named .blacklist and contains keys of type blacklist. If the blacklist
contains a key whose description matches the hash of the new key, that
new key will be rejected with error EKEYREJECTED.
The blacklist keyring may be loaded from multiple sources, including a
list compiled into the kernel and the UEFI dbx variable. Further
hashes may also be blacklisted by the administrator. Note that black-
listing is not retroactive, so an asymmetric key that is already on the
system cannot be blacklisted by adding a matching blacklist entry
The asymmetric key type first appeared in v3.7 of the Linux kernel, the
restriction function in v4.11 and the public key operations in v4.20.
keyctl(1), add_key(2), keyctl(3), keyctl_pkey_encrypt(3),
keyctl_pkey_query(3), keyctl_pkey_sign(3), keyrings(7), keyutils(7)
Linux 8 Nov 2018 ASYMMETRIC-KEY(7)
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